This invention relates to decreasing surface roughness of coatings deposited on glass or equivalent substrates with consequent improvement in optical quality by a reduction in haze. Specifically, the invention reduces the haze typically seen in commercially available doped tin oxide coatings on glass, and particularly those coated in deposition processes requiring high substrate temperatures.
Transparent conductive coatings on glass have a wide range of applications in the electronics and optics industry. Thin films of such coatings are highly transparent in the visible region of the spectrum, have high electronic conductivities and are used extensively in information displays. The most widely used transparent conductors are indium oxide doped with tin (ITO) and doped tin oxide coatings, with doped tin oxide being widely used in displays and as solar coatings. Tin oxide coatings are also widely used in electrochromic devices.
Doped tin oxide is a highly durable oxide coating and is a useful transparent conductor where chemical stability and hardness are required. It can be deposited onto glass using many different techniques, for example, ultrasonically assisted spray, chemical vapor deposition, RF sputtering, and spray coating. Depending on the deposition technique and doping level, specific resistivities in the range of 4.times.10.sup.-4 ohms cm to 1.times.10.sup.-3 ohms cm can be obtained. For vacuum-deposited tin oxide, the dopant is usually antimony with dopant levels being less than five mole percent. However, SnO.sub.2 doped with fluorine is more conductive than the antimony-doped material and is usually deposited by spray deposition. Resistivity as low as 5.times.10.sup.-4 ohms cm and high visible transparency are not unusual for spray deposited fluorine-doped tin oxide.
Tin oxide is available commercially by a spray pyrolysis deposition method, where the deposition is done on the float line as the glass is produced. This is relatively cheap compared to evaporation or sputtering techniques, which makes float line-deposited tin oxide very attractive as a transparent conductor in commercial devices.
A problem with this glass for the applications described above, and many others, however, is their high haze and/or high surface roughness. This haze increases and is particularly noticeable in thick coatings which have been deposited at high temperature onto soda lime glass (or onto SiO.sub.2 barrier-coated glass, or equivalent barrier-coated glass where the barrier coating is insufficiently effective).
For most tin oxides and particularly tin oxides deposited by high-temperature means such as spray pyrolysis, CVD or the like, when the coating material contacts a heated substrate, typically greater than 400.degree. C. or thereabouts and often as high as 500-700.degree. C., and most particularly where the substrate has an exchangeable ion such as sodium which can react with a component of the coating precursor (ie, the chloride in tin tetrachloride), a haze results. This is disadvantageous in windows but is most undesirable in specular mirrors such as rearview mirrors and the like, which can appear cloudy in reflection when tin oxide-coated glass is used as a substrate in a second surface design. Surface roughness contributes significantly to this haze. Although not being bound by theory, we speculate that a lack of intimate contact and/or refractive index mismatch between the rough tin oxide surface and the medium it contacts contributes significantly to the haze appearance. Further, surface roughness increases the difficulty of cleaning the coating surface. Although surface polishing, such as with a cerium oxide slurry or the like, is commonly recommended to reduce the haze seen in tin oxide coatings, and render such coatings more readily cleanable, such polishing is economically disadvantageous and is not entirely effective.
Transparent conductors play an important role in electrochromic devices. Indeed, tin oxide is one of the most widely used transparent conductors in electrochromic devices. For example, U.S. Pat. No. 3,806,229 to Schoot et al. describes single-compartment, self-erasing, solution-phase electrochemichromic (ECC) devices that utilize two glass plates, coated on their inwardly facing surfaces with a conductive layer of tin oxide, that sandwich a solution of an organic solvent such as acetonitrile containing electrochromically-active organic species dissolved therein. Such organic solutions, which typically have a refractive index between 1.3 to 1.6, inherently render usable ECC devices that utilize commercially available tin oxide-coated glass. Mirror and window devices utilizing such construction and solutions inherently have low to negligible haze and are of acceptable optical quality.
However, there are important advantages to using a solid-state, thin-film electrochromic device, instead of a solution phase electrochromic device. For example, the solid-state, thin-film electrochromic construction 1, shown schematically in FIG. 1, as typically applied fails to overcome the optical haze present in most commercially available tin oxide transparent electron conductors. The device shown in FIG. 1 typically consists of an electrochromic working electrode 10 and a counter electrode 20 which can function as either a sink or source of ions for use in coloring the electrochromic working electrode. Both the working and counter electrode are separated by an electrolyte 30 that is ion-conducting but electron-insulating. The two electrodes 10/20 and the electrolyte 30 are sandwiched between two electron conductor layers 40 deposited on substrates 50, which are glass. Layers 40 are connected to a power source 45. For economic reasons, it would be desirable to use high temperature deposited doped tin oxide coatings as conductive layers 40. However, in the case of a transmissive device, both of these conductors 40 must be highly transparent, and in the case of a mirror device one can be reflective. The various electrochromic films 10, 20 and 30 have typically been applied either by sputtering or by vapor deposition. While such films have also been applied by wet chemical deposition techniques, such applications have apparently only been made to tin-doped indium oxide coatings, which have an inherent clarity and lack of haze. The haze associated with tin oxide layers has discouraged their use in electrochromic devices which must be of good optical quality.